My own opinion, I have 2 points here:
1. I agree with the first two comments of since it seems to be out of hand and for me was too much and uncalled for.
2. On the otherhand, You have to accept it that you have also a lapse in your part. This incident won't occur since there is such a thing as "respect" and ethics, bine based ko lang ito from your above mentioned statement.
What you could've done is to purchase the pet first and sell it at a higher price you wanted, this is business and there is a risk involved (wala siguro naging issue) eto problem kapag sobra tayong sigurista eh. I don't uderstand why you didn't inform the seller first what was your plan, your sister was involved in selling cars online? I wonder if she's doing the same thing? posting other cars for sale without knowledge of the direct sellers? hindi naman cguro dba? You should've been aware that a lot of scammer are everywhere, and to be honest, it wasn't your intent to scam and I believe your statement, but somehow, your on your way for a lose-lose situation here, with the pet owner/seller and you. Let's say I'm a real prospective buyer, got interested with your post on pet, then I will go to other fb related groups also and other site and found the same pic with a different price so I'm gonna get confused which is which.... Your sister should've known it since she's also selling online (nag iikutan lang ang gma buyers at sellers and photos like that is a redflag to be honest, benefit of the doubt though na it was honest mistake on your part, pero you have your mistake din, I will get mad either if I am the seller and you did that since it could affect other potential buyer, parang ang labas kasi ginulo nyo lang ung pet na binebenta.
Although I don't condone that act of cyberbullying, may this be a lesson with you and your sister.